Everything Jef has said is great advice, I would add simply accept that there are times when you will need to disconnect the focus and that's ok.

There are literally thousands of different methods, models, theories, strategies on this. For what it is worth here is another one that is simple but extremely effective. You do need the discipline though.

First, I would say that this isn't unique to you. There are more distractions today than I could ever mention. However, there are definite ways to overcome or at least minimize them.

Determine what time of the day you are most productive (morning, mid-morning, afternoon, evening, late-night). I won't bore you with details but this is probably when you are in an optimized period of "Flow". I've included a link to the book if you have more interest in learning about it.

Once you understand personal flow, attack your tasks in focus (related to perceived impact upon completion, priority or time needed to accomplish). This makes a difference. But why? you can move less important tasks (checking email, following up on voicemail, organizing files, etc.) for times you aren't in optimal flow.

Also, I've written an article on here related to this called, "3 + 1 = Improvement". I highly encourage you to read that to extend the information above further.

Remember processes and routines? Sure they are less spontaneous, but they bring efficiency for reducing time and mental load. Document the processes you use so that you can make them routine. This will help reduce the feeling of having to "mentally reconnect" each time.

Let me know if you would like any more detail on the information above or if you have further questions.